When fuel injection is carried out by an injector, a computer (ECU) determines the amount of fuel to be supplied in one combustion cycle on the basis of factors such as amount of an accelerator pedal stroke, engine speed and the like.
In the case of an engine having a plurality of cylinders, an injector is arranged in each cylinder, so that a respective injector injects fuel into the corresponding cylinder. Therefore, individual variability in the injectors is reflected as differences in performance among the cylinders. Specifically, even when an injection amount instructed from a computer to the injector is constant, the injection amount actually injected by the injector shows variations, albeit small variations, among individual injectors, which results in output energy differences among cylinders.
On the other hand, in order to suppress engine vibration, the angular velocity of the crankshaft is preferably constant, irrespective of the rotation angle. To that end, the output energy difference among cylinders needs to be cancelled, and hence it is necessary to equalize the amount of fuel actually injected into each cylinder. The instructed injection amount is thus adjusted among cylinders. This is called inter-cylinder adjustment. Specifically, the injection amount is reduced for a cylinder having a faster angular velocity than other cylinders, and is increased for a cylinder having a slower angular velocity than other cylinders, to correct thereby the injection amount in one combustion cycle for each cylinder, in such a way that the angular velocities of the cylinders become identical.
That is,QFINR=QFIN+Acyl holds for a given cylinder, wherein QFIN denotes an injection amount before correction, Acyl denotes an inter-cylinder correction amount, and QFINR denotes an injection amount after correction, in one combustion cycle.
Among individual engines, owing to variations in the performance of the engine itself and/or to injector variation, engine output varies even for an identical injection amount instructed by an ECU. The correction for equalizing engine output is the output correction (Q adjustment correction). Specifically, that correction involves adjusting the injection amount in such a way that that full load output becomes constant. Full load designates the portion having the largest output in each engine speed, and full load output denotes the output at that time.
Herein,QFINR=QFIN+Apow holds for a given cylinder, wherein QFIN denotes an injection amount before correction in one combustion cycle, Apow denotes an inter-cylinder correction amount, and QFINR denotes an injection amount after correction.
Also, fuel volume varies with temperature. That is, when volume increases due to thermal expansion, a desired mass of fuel fails to be injected even if the injector injects a certain volume of fuel. Therefore, a correction is carried out in which the fuel injection amount at volume is adjusted in such a way that the fuel injection amount at mass does not vary due to fuel temperature.
When all these different corrections are carried out, the total correction amount is the sum of the respective correction amounts.